There are 75 playable vehicles in FUEL, each with its own set of properties and features. You can change the vehicle at any time by accessing the Garage in the Pause menu, and buy new ones depending on how much FUEL (the games currency) you have to spend.
The key to winning is to have a variety of vehicles at your disposal to cope with all the challenges that will come your way. A dirt bike will be nimble and able to cope with punishing off-road terrain, but it is not going to be much help to you in a long asphalt road race against bigger more powerful vehicles. Before selecting a vehicle, you can view its attributes to decide if it is the right one for you.
Also, even though two vehicles of different type may have the same stats, the category of vehicle impacts how they differ and perform. EG: a buggy and truck may have the same stats, but the buggy will be lighter, accelerate a bit faster, gain more air on jumps (and thus slow down more due to air time) and perhaps skid more on turns while the truck will be slower to accelerate, and hug the landscape and corners more due to its greater weight.
Unlocking some zones can unlock newer vehicles. Pay the FUEL to get your ride. (Get your FUEL by hitting Fuel Barrels, or winning races & challenges.)
A few races offer a new ride as part of your prize.
Several races unlock a roaming vehicle ( purple icon) in the current zone. Chase it down, ram into it, and it's yours.
Mavericks can be entertaining or infuriating to chase down, depending on how far away they are from nearest helipad and how fast they're travelling. They're always worth getting, though, because they're generally superior to any equivalent vehicle you can get in the game at that time plus they're free. Win-Win.
Max speed of vehicle on its optimum surface. Look at this stat in conjuction with Acceleration, because acceleration helps the vehicle get to its top speed faster. A vehicle with great top speed but poor acceleration may never see its top speed maxed out as it can't reach it before it has to slow down around another corner or obstacle. Max speed is reduced when vehicle is running on sub-optimum surface (eg: street vehicle running on dirt-road or off-road).
How quickly you can get off the line, and up to top speed. Road type impacts acceleration in that a vehicle with high acceleration can get stuck simply spinning its wheels and hitting its throttle limiter when on a road it's not optimized for (eg: road bike that's gone off-road).
How 'sticky' the vehicle is on the road, which means how much it can corner before it starts drifting / sliding / skidding, and how much control you have when it does. Grip also affects hill-climbing, as good grip helps your wheels dig in and get the acceleration going to get up the hill. A vehicle's surface specialization impacts grip (like speed & acceleration). Street vehicles may have lots of grip on a road, but less on dirt-roads and hardly any off in the brush. Off-road vehicles have lots of grip on roads & don't drift as much when off-road (but handicap by having lower top speeds that street vehicles).
How quickly you can bring your ride to a stop. Probably the most useless stat, because just letting off the gas to coast is all you need to do to make it around almost any corner or obstacle. Normal & hand-braking is fun for skidding out during joy-rides, but you'll probably never use them for racing.
Resistance to damage, shown in bottom-right corner of screen next to total mileage, revs and speed. Seems greatly tied to vehicle mass/weight/size, so also a good indicator of how well vehicle will hug terrain vs. flying when hitting hills & bumps. EG: Motorcycles go flying off hills while monster trucks seem glued to the terrain even on hills. (This is important to point out, because FUEL has a horrible air friction that causes drastic speed reduction during jumps. As such, while it's fun to ramp in free-ride, it's best to keep your wheels on the ground during races to maintain speed and momentum. That's why even when running an off-road vehicle it's best to stick to roads, as vehicles hug roads while off-road terrain tends to have bumps and jumps that can send a vehicle flying.)
How well vehicle performs on asphalt roads/streets. Specializing vehicles have better grip, acceleration and top speeds on asphalt. They can still perform on dirt roads, but grip, acceleration and top speed suffer some. They should avoid off-road as some take a drastic hit to their performance.
How well vehicle performs off-road (and by extension, dirt-roads). "Mixed" vehicles do well on dirt roads, and can handle some off-road duty (but not as well as pure off-roaders). Pure off-roaders do well on any road type, but don't magically gain a lot more top speed on asphalt the way pure asphalt and, to a lesser extent, mixed types do.